Closed refrigerant cycle for the liquefaction of low-boiling gases

ABSTRACT

In a process for the liquefaction of a cryogenic gas such as helium, comprising cooling and liquefying said gas by indirect heat exchange with a refrigerant circulating in a closed refrigeration cycle, the improvement comprising conducting said heat exchange with the refrigerant from a single refrigerant cycle, said refrigerant being subjected to both engine expansion and at least partially isenthalpic expansion, whereby the refrigerant is cooled sufficiently to effect liquefaction of all the cryogenic gas in a single pass, thereby avoiding the necessity of additional compressor or purification capacity for recycled gas.

United States Patent [191v Spies et al.

[111 3,828,564 [451 Aug. 13, 1974 late of Munich, all of Germany by Anne-Rose Sellmaier, Co-Heir [73] Assignee: Linde A.G., Hollriegelskreuth,

Germany [22] Filed: Mar. 1, 1971 [21] Appl, No.: 122,605

[52] US. Cl. 62/9, 62/40 [51] Int. Cl... F2Sj 1/00 [58] Fieldof Search 62/9, 11, 22, 38, 40, 191,

- [56] g I References Cited UNITED STATES PATENTS 3,098,732 7/.1 963 Dennis 62/9 3,152,457 10/1964 Zotos 62/500 3,180,709 4/1965 Yendall 62/38 3,194,025 7/1965 Grossman 62/40 1 3,250,079 5/1966 Davis 62/40 3,389,565; 6/1968 Ergenc 62/9 3,434,298 3/1969 Rietdijk 62/500 3,447,339 6/ 1969 Rietdijk 62/500 3,456,456 7/1969 Rietdijk 62/500 3,496,735 2/1970 Haisma 62/500 3,557,566 1/1971 Van Den Born 62/9 3,613,387 10/1971 Collins 62/9 FOREIGN PATENTS OR APPLICATIONS 987,569 3/ 1965 Great Britain 62/40 Primary Examiner-Norman Yudkoff Assistant Examiner-F. Sever Attorney, Agent, or Firm-Millen, Raptes & White [5 7] ABSTRACT all the cryogenic gas in a single pass, thereby avoiding the necessity of additionalcompressor or purification capacity for recycled gas.

4 Claims, 4 Drawing Figures PATENTEB ms 1 31974 SHEEI 1 0f 4 ATTORNEY PATENIE MIR 1 31974 SHEU 2 0f 4 INVENTOR z ATTZRNEY PATENTED AUG 1 3 m4 SHEET 3 BF 4 T I i l I l l l l l l i l l l 2 ATTORNEY PATENIE ms 1 3 m4 SHEEI t Of 4 INVENTOR 1 CLOSED REFRIGERANT CYCLE FOR THE LIQUEFACTION OF LOW-BOILING GASES BACKGROUND OF THE INVENTION uncondensed gaseous portion, which remains after the J-T expansion is recycled to the entering stream of raw gas downstream of the compressionstep where it is reintroduced, together with raw gas stream, through the purification units. During this procedure, however, the

order to obtain the same purification otherwise attain able in smaller units without the recycled gas. This scale-up problem cannot be economically solved by mixing the unliquefied fraction of purified gas with the mainstream of gas to be liquefied at a point downstream of the purification units because'another compressor would be required to compensate for the pressure difference necessary for the J-T expansion. Thus, in any case, an additional expenditure for apparatus is inherent if the gaseous stream fed into the apparatus to be liquefied is only partially liquefied.

Another disadvantage of J-T expansion is that a high level of compression must be employed. This high pressure is in considerable excess of the pressure required for overcoming resistance to flow. This means, for example, that raw gas to be fed from pressurized storage bottles cannot, by and large, be introduced into the liquefying plant.

As an alternative to J-T expansion, it is also known that the gas to be liquefied can be cooled to below the inversion point by a closed refrigeration cycle embodying repeated engine expansion of a cooling medium. The gas to be liquefied, precooled, in this manner, is condensed by heat exchange'with another coolant, the latter providing the refrigeration necessary for the final liquefaction and being cooled in a separate closed refrigeration cycle by throttle expansion. Consequently, in this process, there is also a considerably increased cost associated with the additional compression unit required for this separate refrigeration cycle.

SUMMARY OF THE INVENTION A principal object of this invention is to provide an improved process and apparatus therefor for the liquefaction of low-boiling gases, and especially a system which is substantially devoid of above-mentioned disadvantages, thereby resulting in relatively minimum investment for the required apparatus. 7

Upon further study of the specification and appended claims, other objects and advantages of the present invention will become apparent.

To attain the objectives of this invention, the production of refrigerant energy is based on a single refrigeration cycle wherein: (a) in at least one branch, an engine expansion is conducted; and (b) in one or more parallel-connected branches, an at least partially isenthalpic expansion is conducted.

In the simplest case. operating according to the invention, an engine expansion is conducted in one branch of the refrigeration cycle, for example by means of an expansion turbine, and an isenthalpic expansion is conducted in a branch connected in parallel therewith. It is also possible though, and in fact advantageous, to provide a plurality of engine expansions in combination with a plurality of isenthalpic expansions.

Aside from conducting a single or repeated engine expansion of the cooling medium in appropriate branches of the refrigeration cycle, another feature of this invention provides forexpanding the cooling medium by means of an ejector and an expansion valve in a branch disposed in parallel with respect to the other branches.

The operation of the above-described processes can be even further modified in accordance with a particular embodiment of the invention, i.e., by an isenthalpic expansion of the gas to be liquefied, wherein the proportion of the gas which is not at first liquefied is liquefied by heat exchange with the cooling medium.

It is also provided that for the optimum utilization of the available refrigeration energy the liquefied refrigerant gas, after re-evaporation is brought in heat exchange with the gas to be liquefied.

A particularly advantageous feature of this invention is, inter alia, that it is possible to liquefy a cryogenic gas, even helium, with a low expenditure in apparatus and wherein the ratio of the liquefied pure gas to the introduced pure gas reaches unity. Furthermore, because it is possible to do without an isenthalpic expansion of the gas to be liquefied, the selection of the feed pressure is dependent solelyon those pressure conditions determined by the optimum effect of the purifica tion devices, by the given flow resistance, and by the final pressure to be maintained in the liquefied gas collecting tank. In case of only minor impurities in the raw gas, the pressure, which is required mainly for the adsorptive purification, can be very much lower than the pressure used previously in liquefaction processes known heretofore. A further significant advantage of the present invention is that it is possible to conduct the compression required in the refrigeration cycle branches for the engine expansion and/or isenthalpic expansion by means of one single compressor.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings illustrate preferred embodiments of this invention as applied to helium liquefaction, wherein:

FIG. 1 is a schematic representation of a simple embodiment of the invention.

FIG. 2 is a schematic embodiment of a liquefaction plant according to this invention exhibiting several expansion stages in the refrigeration cycle.

FIG. 3 schematically depicts a liquefaction plant having an ejector stage in addition to the apparatus illus trated in FIG. 1.

FIG. 4 is also a schematic simplified representation of a liquefaction plant which, as compared to the embodiment shown in FIG. 1, permits the gas to be liquefied and to be treated in a different way.

DETAILED DESCRIPTION OF THE DRAWINGS According to FIG. 1, helium, purified externally of the liquefaction apparatus, is fed under low pressure through liquefaction system and condensed in a collector tank. The liquefaction system contains as the essential elements, the flow systems for the refrigerant and the main section of the flow system for the helium to be liquefied, the latter being designated hereinafter as process helium. The raw process helium passes, for example, from a pressurized cylinder 12, via conventional purification means 13 for the removal of water, oil, and CO and via an air separator 14, in the form of pure gas into the liquefaction system 20. The resultant process gas is then throttled via valve 15, to that necessary to compensate for the fluid flow pressure drop and to yield the desired final pressure in collector tank 10. The pure process helium flows successively through the heat exchangers 2, 3, 4, 5, and 6 and finally is condensed in the condensation heat exchanger 7 positioned preferably in the collector tank. The effluent liquid process helium collected in the lower portion of the collector tank 10 is withdrawn via the outlet valve The heat removal necessary for cooling and condensing the process helium is effected by employing circulating helium as the refrigerant in the closed refrigeration cycle. This helium is compressed by the compressor 1 and precooled in the heat exchangers 2, 3, and 4; this precooling step is conducted in heat exchangers 2 and 4 with the recycled expanded helium of the refrigeration cycle, but in heat exchanger 3 disposed between the heat exchangers 2 and 4, for example, by a refrigeration cycle 16 with liquid nitrogen. The helium precooled and compressed in this manner is now in part, about 40 to 80 percent, fed to the expansion machine 9, in order to absorb heat by a substantially isentropic expansion. After engine expansion, the helium is passed through heat exchangers 5, 4, and 2, and recycled to the compressor 1. The other portion of the precooled and compressed helium passes into the heat exchanger 5 and is further cooled therein by the helium cooled in the expansion machine 9.

The further cooled helium withdrawn from heat exchanger 5 is then passed into the following heat exchanger 6, where the temperature is lowered to a value below the inversion point optimum for the liquefaction.

The gas is then subjected to a .I-T expansion by passing through valve 8. This results in partial liquefaction of the refrigerant which is then passed to condensation heat exchanger 7 where it is re-evaporated, providing the necessary heat removal required for the liquefaction of the process helium in the other flow system.

The re-evaporated helium of the refrigeration cycle, after leaving the condensation heat exchanger 7, traverses heat exchanger 6, and cools the precooled helium introduced therein, as well as the proportion of helium of the respective branch of the refrigeration cycle intended for the Joule-Thomson expansion, to a temperature value below the inversion point. Finally, that helium which either was not liquefied or which was re-evaporated is combined with the engine-expanded proportion and the combined stream is then passed through the heat exchangers 5, 4 and 2 to transfer the refrigerant values therein and then recycled to compressor 1.

Depending on the purpose for which the plants are used, the liquefied gas is discharged accordingly. In case of small plants, the liquefied gas is discharged into storage vessels or transport containers 17, e.g., tank cars or trucks In case of large plants, storage tanks are usually employed for the liquefied gases, and where an automatic, unsupervised continuous operation is employed, it is desirable to employ storage tanks having a high capacity. I

With respect to the process conditions in FIG. 1, the pressure of the process helium in the liquefaction system downstream of valve 15 is usually more than 1 atmosphere absolute. The pressure of the refrigerant helium derived from compressor 1 is usually more than 18 atmospheres; the pressure after engine expansion in 9 is about 1 to 1.5 atmospheres absolute; the pressure drop through the J-T expansion valve is about 15 to 30 atmospheres and the pressure after J-T isenthalpic expansion in valve 8 is about 1 to 2 atmospheres. The

temperature of the evaporating refrigerant helium in condenser 7 is about 42 to 5.0 K.

Referring now to FIG. 2, it can be advantageous to pass process gas to be liquefied through the expansion valve 21 illustrated in dashed lines, to subject said process gas to an isenthalpic expansion prior to the final liquefaction. However, contrary to conventional practice, the proportion which remains in the gaseous phase is not recycled countercurrently to the incoming pure process gas through the corresponding heat exchangers. Instead, this gaseous proportion is passed into the condensation heat exchanger 7 and is entirely liquefied therein. Thus, by this mode of operation, there is eliminated the scale-up problem mentioned in the Background of the Invention, namely the necessity of scaling-up the purification system to accommodate recycle process gas. Only the feed pressure must be adjusted .and controlled by the valve 15, thereby obtaining a steady pressure from the gradually diminishing pressure in the pressurized cylinder 12. Where cylinders are not employed, any compression device necessary for producing the pressure required for the isenthalpic expansion and for overcoming the flow resistance would only have to compress, also in this case, the amount of process gas actually liquefied in one pass through the liquefaction system, rather than compress additional amounts of recycle unliquefied pure process gas.

FIG. 2 also exemplifies a plant provided with two stages of engine expansion, 9a and 9b, and two stages of isenthalpic expansion, 8a and 8b.v From the standpoint of thermodynamic efficiency, this embodiment of the liquefaction plant having several expansion stages can be more advantageous than a liquefaction plant operating with only one-stage expansion of the refrigerant.

Referring now to FIG. 3, a further improvement of the process of this invention is obtained by expanding the helium circulating in the refrigeration cycle through an ejector 22, as well as through the simple throttle valve 8. In addition to obtaining the conventional advantages of a resultant lower energy requirement for the compression of the coolant circulating in the refrigeration cycle (at the same final pressure, the suction pressure of the compressor 1 can be raised, which permits the use of a smaller compressor), there is the possibility, when employing an ejector, of lowering the condensation temperature for the process helium to be liquefied. Accordingly, the condensation conditions are improved by having the temperature of evaporating helium in the condenser 7 at a temperature of 3.5 to 4.0 K. In this way the liquid helium can be subcooled to such an extent that, wh en being transferred into transport or storage vessels, there are no appreciable evaporation losses of the liquid.

The ejectors which can be employed are conventional and for further details, attention is invited to Handbuch der Kaeltetechnik, Band 2, 1953, Seite 373.

In FIG. 3, re-evaporated refrigerant, after traversing heat exchanger 6c and having a temperature of about 3.5 to 4.2 K. and a pressure of about 0.4 to 1.0 atmospheres absolute, is mixed with compressed refrigerant withdrawn from exchanger 6 having a temperature of about 5 to 7.5 K. and a pressure of about 10 to 30 atmospheres absolute, in ejector 22 where the outlet conditions of the latter are 4.4 to 5 K. and 1.2 to 2 atmospheres absolute. The ejected fluid is fed to a phase separator, from where the liquid is vaporized in exchanger 6c, the resultant gas being passed through J-T valve 8. The gas from the phase separator which amounts to about 70 to 90 percent by weight of the helium entering the phase separator is recycled to compressor 1, as in FIG. 1.

Referring now to FIG. 4, if a liquefaction plant according to this invention is coupled with storage vessels or tanks 17 disposed on the outside, the degree of efficiency of the entire plant can be improved by recycling through the liquefaction system the cold vapor unavoidably produced when discharging the liquid into the tanks or storage vessels 17. Thus, the cold vapor flowing back from the tank or storage vessel 17 to the liquefaction apparatus is warmed to ambient temperature in heat exchange with the process gas to be liquefied. Because of the additional cooling energy obtained, a larger amount of gas can be liquefied. The amount of process gas produced by the unavoidable reevaporation and then recycled through the liquefaction system can be fed to a central recovery system 18. If no such system is present, then it is advantageous, in case of expensive gases, to provide a relatively small compressor for compressing said gas to the feed pressure of the gas to be liquefied.

Referring again to the process according to FIG. 1, provision is made to purify the process gas in the air separator 14 prior to entering the liquefaction apparatus 20. Though this technique is suitable in small plants, with certain conditions regarding the extent of impurities, in case of larger plants, it is advantageous to arrange the purification devices within the liquefaction apparatus 20. In FIG. 4, the location of such purifying devices is exemplified by the devices 14a and 14b.

Whereas this invention has been described in detail in connection with the liquefaction of helium, it is also useful for the liquefaction of other cryogenic gases, e.g. hydrogen, methane, nitrogen, neon, etc.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

All atmospheres recited herein are absolute.

What is claimed is:

1. In a process for the liquefaction of a cryogenic gas by indirect heat exchange with an at least partially liquefied refrigerant which is evaporated and warmed during this heat exchange, the partial liquefaction of the refrigerant being conducted in a closed refrigerating cycle having a precooling stage and a low tempera ture cooling stage, wherein, in the precooling stage, the refrigeration is produced by engine expansion of at least one compressed partial stream of the refrigerant, and in the low-temperature cooling stage, the refrigeration is produced by multistage isenthalpic expansion of the residual stream of the compressed refrigerant which has not been engineexpanded to produce said at least partially liquefied refrigerant, the improvement comprising:

a. conducting at least one isenthalpic expansion of the residual refrigerant which has not been engineexpanded in the low-temperature stage with an ejector;

b. passing resultant cooled expanded, residual refrigerant to a phase separator and separating liquefied refrigerant therein;

c. vaporizing resultant liquefied refrigerant in indirect heat exchange with cryogenic gas;

d. passing resultant vaporized refrigerant through a throttle valve to isenthalpically expand and partially liquefy the refrigerant;

e. passing resultant partially liquefied refrigerant in further indirect heat exchange with the cryogenic fluid cooled and withdrawn from step (c) to vaporize said partially liquefied refrigerant and to complete the condensation and subcool said cryogenic fluid;

f. passing resultant vaporized refrigerant in indirect heat exchange with the cryogenic gas to be liquefied; and

g. passing resultant warmed vaporized refrigerant to the suction side of the ejector to lower the vaporization temperature of the refrigerant, thereby permitting the subcooling of the liquefied cryogenic gas,

and with the further provision that the cryogenic gas is liquefied without an isenthalpic expansion thereof during its liquefaction.

2. Aprocess according to claim 1, there being conducted a plurality of said engine expansions, and at least two series-connected stages ofsaid isenthalpic expansions.

3. A process according to claim 1, further comprising storing resultant liquefied gas under conditions wherein said liquefied gas is re-evaporated, and passing said reevaporated liquefied cryogenic gas into countercurrent heat exchange with the cryogenic gas to be liquefied.

lium.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated August 13, 1974 Patent No. 3I828!564 Inventor(s) Anton Spies, et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE HEADING:

Insert Claims priority, application Germany P 20 09 401.3,

February 27, 1970 Signed and sealed this 29th day of October 1974.

(SEAL) Attest:

C. MARSHALL DANN McCOY M. GIBSON JR. Attesting Officer Commissioner of Patents USCOMM-DC GOING-P69 0.5 covsmmzm PRINTING OFFICE: n0 aqua-3:4,

FORM PO-105O (10-69) 

2. A process according to claim 1, there being conducted a plurality of said engine expansions, and at least two series-connected stages of said isenthalpic expansions.
 3. A process according to claim 1, further comprising storing resultant liquefied gas under conditions wherein said liquefied gas is re-evaporated, and passing said re-evaporated liquefied cryogenic gas into countercurrent heat exchange with the cryogenic gas to be liquefied.
 4. A process as defined by claim 1 wherein both the refrigerant and the cryogenic gas to be liquefied are helium. 